Carolina Panthers quarterback Cam Newton (1) is chased by Chicago Bears defensive end Julius Peppers (90) and linebacker Brian Urlacher (54) in the second half of an NFL football game in Chicago, Sunday, Oct. 2, 2011.Associated Press

The Bears' defense will have an opportunity Monday night in Detroit to show sold-out Ford Field and a national TV audience that it isn't as bad as it seems on paper or as bad as it has looked on the field.

Last season's defense was No. 4 in points allowed, but this year it's No. 20. A year ago, the Bears were No. 2 in rushing yards allowed; this year they're No. 31.

"That's a complete 180," safety Chris Harris said. "Everybody's taking a step back and looking at themselves in the mirror. We don't point any fingers around here. We just have to get better individually (and) as a group."

The personnel is essentially the same, although Harris missed the previous three games with a hamstring injury. So what's wrong with the Bears' defense?

"I just think we've had some breakdowns fundamental-wise, overshooting some tackles," defensive coordinator Rod Marinelli said. "For us to get back on track, that has to (improve). They know it, and we know it."

Opponents have scored at least 27 points in three straight games. The Bears permitted that many points just twice all last season, and not at all in the first 12 games.

Every 2011 opponent has at least 382 yards; that happened just four times in 2010.

"We've given up a lot of points and yards, which isn't a good thing," linebacker Brian Urlacher said. "Luckily we've managed to win two of those games. We're just not making big plays. We're giving up too many long runs, too many long passes. We're giving up big plays and not making any ourselves. We have to figure out how to get back to the way we used to play."

The Bears really have missed the big play.

They've intercepted just 3 passes, and Urlacher has 2. No one who has started a game in the secondary this season has a single interception. Nickel corner D.J. Moore's pick last week is the only one by a defensive back.

Lance Briggs leads the Bears with 46 tackles, but just 2 have been for negative yardage, and the Bears have just 9 tackles for loss as a team (2.25 per game). Last year they had 49 (3.06 per game). And the Bears are 25th in the league with just 8 sacks.

"It hasn't been typical of the type of defense we've played or that we're supposed to play to be successful or to be one of the top defenses in the league," Briggs said. "When you turn the tape on, you see the mistakes. Guys aren't in their gaps; guys aren't playing as violently as they need to play.

"It starts right here," Briggs said, pointing at himself. "When we do that -- and we'll do that this week -- we'll get back on track."

The defense has already allowed 17 pass plays of 20 yards or more, on pace for a total of 68. Last season it allowed just 44. Run defense is almost as bad, big play wise. In 2010, opponents had 45 runs of 10 yards or more. But the Bears have already allowed 14 this year, on pace for 56.

"We just have to get back to the basics, hustling and doing what we do best, which is flying around to the football, having fun and just feeding off of one another," nose tackle Anthony Adams said. "I think we'll do that with our remaining games.

"We need to hustle more and get off blocks. Usually if somebody makes a mistake, there's somebody else hustling to correct that mistake. That hasn't been the case here lately. I think guys are playing too uptight; we just have to let it loose."

The sooner the better. After just the first quarter of the season, the Bears are already running out of time. A loss Monday would leave them 3 games behind the Lions and maybe the Packers, too.